The certificate specified by the InstantMessagingCertificateThumbprint parameter of the Outlook Web App virtual directory ...

The certificate specified by the InstantMessagingCertificateThumbprint parameter of the Outlook Web App virtual directory will expire on %1. When this certificate expires, users won't be able to access the Instant Messaging service through Outlook Web App. Please renew the certificate and update the InstantMessagingThumbprint parameter on the Outlook Web App virtual directory with the thumbprint of the new certificate.